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      1 // Copyright 2014 The Chromium Authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 #include "sandbox/linux/services/thread_helpers.h"
      6 
      7 #include <errno.h>
      8 #include <signal.h>
      9 #include <sys/types.h>
     10 #include <sys/stat.h>
     11 #include <unistd.h>
     12 
     13 #include <string>
     14 
     15 #include "base/basictypes.h"
     16 #include "base/logging.h"
     17 #include "base/posix/eintr_wrapper.h"
     18 #include "base/strings/string_number_conversions.h"
     19 #include "base/threading/platform_thread.h"
     20 #include "base/threading/thread.h"
     21 
     22 namespace sandbox {
     23 
     24 bool ThreadHelpers::IsSingleThreaded(int proc_self_task) {
     25   CHECK_LE(0, proc_self_task);
     26   struct stat task_stat;
     27   int fstat_ret = fstat(proc_self_task, &task_stat);
     28   PCHECK(0 == fstat_ret);
     29 
     30   // At least "..", "." and the current thread should be present.
     31   CHECK_LE(3UL, task_stat.st_nlink);
     32   // Counting threads via /proc/self/task could be racy. For the purpose of
     33   // determining if the current proces is monothreaded it works: if at any
     34   // time it becomes monothreaded, it'll stay so.
     35   return task_stat.st_nlink == 3;
     36 }
     37 
     38 bool ThreadHelpers::StopThreadAndWatchProcFS(int proc_self_task,
     39                                              base::Thread* thread) {
     40   DCHECK_LE(0, proc_self_task);
     41   DCHECK(thread);
     42   const base::PlatformThreadId thread_id = thread->thread_id();
     43   const std::string thread_id_dir_str = base::IntToString(thread_id) + "/";
     44 
     45   // The kernel is at liberty to wake the thread id futex before updating
     46   // /proc. Following Stop(), the thread is joined, but entries in /proc may
     47   // not have been updated.
     48   thread->Stop();
     49 
     50   unsigned int iterations = 0;
     51   bool thread_present_in_procfs = true;
     52   // Poll /proc with an exponential back-off, sleeping 2^iterations nanoseconds
     53   // in nanosleep(2).
     54   // Note: the clock may not allow for nanosecond granularity, in this case the
     55   // first iterations would sleep a tiny bit more instead, which would not
     56   // change the calculations significantly.
     57   while (thread_present_in_procfs) {
     58     struct stat task_stat;
     59     const int fstat_ret =
     60         fstatat(proc_self_task, thread_id_dir_str.c_str(), &task_stat, 0);
     61     if (fstat_ret < 0) {
     62       PCHECK(ENOENT == errno);
     63       // The thread disappeared from /proc, we're done.
     64       thread_present_in_procfs = false;
     65       break;
     66     }
     67     // Increase the waiting time exponentially.
     68     struct timespec ts = {0, 1L << iterations /* nanoseconds */};
     69     PCHECK(0 == HANDLE_EINTR(nanosleep(&ts, &ts)));
     70     ++iterations;
     71 
     72     // Crash after 30 iterations, which means having spent roughly 2s in
     73     // nanosleep(2) cumulatively.
     74     CHECK_GT(30U, iterations);
     75     // In practice, this never goes through more than a couple iterations. In
     76     // debug mode, crash after 64ms (+ eventually 25 times the granularity of
     77     // the clock) in nanosleep(2).
     78     DCHECK_GT(25U, iterations);
     79   }
     80 
     81   return true;
     82 }
     83 
     84 }  // namespace sandbox
     85